The present invention relates to fluid speed indicators and, more particularly, to fluid speed indicators that are sensitive to very low fluid velocities and is an improvement of U.S. Pat. Nos. 3,373,605, 3,400,584 and 3,726,139. In these earlier patents, the disclosed airspeed indicators comprise two rotating pitot or venturi tubes which sense the cyclic variations in pressure as the tubes move into and away from the direction of flight. In these devices the variations are proportional to the forward velocity or airspeed of the vehicles to which the indicators are attached.
With the apparatus of the above-mentioned patents the measurements result in a sinusoidally modulated pressure signal at the rotor rotational frequency. In the resolving process, this signal is multiplied by the sine and cosine components of the rotational angle. The resulting outputs are two sinusoids which are of the form EQU V sin.sup.2 .omega. t = (V/2) + (V/2) (sin 2 .omega. t)
The oscillatory output, sin 2 .omega. t, must be filtered out to obtain the steady V/2 terms. This filtering restricts the bandwidth of the measurement.
Additionally, with the apparatus of the above-mentioned patents, the rotor for rotating the sensing tubes must be driven at a relatively high rotational speed to insure a suitable response time. This necessitates higher rotor drive power requirements which increases the tendency for centrifugal pumping or whirl effects that have to be corrected, as by the addition of airfoils. Thus a choice of reducing the speed of the arms supporting the tubes and thereby reducing the power requirements and whirl effects as well as eliminating the need for airfoils is given at a price of lowering the response time of the instrument with the lowering of the RPM.